It is known that nitrates may be physiologically harmful to humans. Over the years, this problem has become more chronic with the extensive use of nitrate-containing fertilizers. As a result of heavy use of nitrate-containing fertilizers there has been gradual contamination of ground water with nitrates.
It can be seen, therefore, that especially in agricultural areas the presence of nitrate in municipal water supplies has become an ever increasing pollution problem. There is, therefore, a continuing need for effective nitrate removal.
There are two major problems with commercially available anion exchange resins that are used for nitrate removal. If sulfate is present in a nitrate contaminated water, a proportionate amount of resin capacity is consumed by the sulfate. Secondly, unless the process is carefully monitored, the concentration effect for nitrate is observed. This phenomenon occurs as resin capacity becomes exhausted. Influent sulfate will then exchange for the previously removed nitrate, causing nitrate levels in the effluent water to actually rise well above the original nitrate concentration. Hence, in a situation where careful monitoring is impossible, effluent nitrate levels will be unpredictable and often the net removal of nitrate very small. This often occurs with a trimethylammonium resin such as DOWEX-1.RTM..
For examples of uses of tributylamine resin, see Guter, U.S. Pat. No. 4,479,877 issued Oct. 30, 1984 and Guter, "Removal of Nitrate From Contaminated Water Supplied for Public Use: Final Report," EPA-600/S2-82-042 of August 1982. The resins described in the Guter patent and EPA publication have some of the problems earlier described herein, namely, reduced capacity and variability of nitrate removal due to sulfate content. This is simply another way of saying that the life of the resin is short and unpredictable with trialkylamine resins. This is illustrated by the examples shown below.
Another problem which exists with use of ion exchange resin techniques for removal of nitrate is that most ground water contains both nitrate and sulfate. Most of the resins used are not highly nitrate selective resins, and in particular will selectively remove sulfate over nitrate. Such renders the resin less useful for nitrate removal. It is therefore desirable to have a resin which is nitrate selective and relatively immune to interfering ions normally present in ground water, such as sulfate. As Guter reports, the presence of significant amounts of dissolved sulfate ion (about 50 ppm or more) has been demonstrated to be an impediment to nitrate removal.
While the Guter technique has involved some ability to selectively remove nitrate with anion exchange resins, the resin has a relatively small selectively coefficient and capacity in comparison with the resins of the present invention, and a shorter life before regeneration is necessary.
It therefore can be seen that there is a real and continuing need for inexpensive anion exchange techniques which can be used with municipal water supplies to selectively remove nitrate from ground water, which may contain anions that are normally regarded as interfering with nitrate removal, such as sulfate. This invention has as its primary objective the fulfilling of this need.
As a result, applicants are able to achieve successful nitrate ion removal, even in high sulfate ion containing water supply. The method and manner of accomplishing each of the above objectives will become apparent from the detailed description of the invention which follows hereinafter.